This invention relates to a method of detecting abnormalities in input or output units connected to an electronic control unit.
The input units mentioned in this specification refer to such devices as various sensors for sensing a variety of parameters which indicate the operation of a controlled object and which are to be processed by an electronic control unit to which the sensors are connected. The output units mentioned in this specification refer to actuators controlled by control signals produced as outputs by the electronic control unit on the basis of, e.g., parameter values obtained from the foregoing sensors. By way of example, where the controlled object is an electronic control system for controlling the operation of an internal combustion engine, the input units are, e.g., various sensors for sensing a variety of engine operating parameters such as engine rpm, intake pipe pressure, engine temperature and throttle valve opening. Examples of the output units are a fuel injection valve, a supplementary air quantity control valve for bypassing a throttle valve, an air-fuel ratio control valve, and various other control devices.
Output signals from input devices of the above-described type are processed by the electronic control unit, and the output unit is actuated by a control output from the electronic control unit commensurate with the results of the processing, thereby controlling the operation of the controlled object. Proper control cannot be performed if an input or output unit develops an abnormality of any kind. Accordingly, in order to avoid a situation in which the electronic control unit operates erroneously owing to an abnormality in an input or output unit, a method adopted conventionally is to sense any abnormality in the input and output units and, when an abnormality is sensed, to either issue an alarm or perform an operation to compensate for the failure.
According to this method of detecting an abnormality in an input or output unit, the method in many cases is as disclosed in the specification of Japanese Patent Provisional Publication No. 141926, which includes obtaining a signal indicative of the operating state of the input or output unit, monitoring the level of the signal, rendering a decision that the input or output unit is operating abnormally when the signal exceeds the upper and lower limits of a normal level continuously for a predetermined period of time, and storing, in a memory device comprising a random-access memory or the like, information indicative of an abnormality for determining whether or not a predetermined alarm action or a predetermined failure compensating action is to be carried out when the abnormality is detected.
With this conventional method of abnormality detection wherein an abnormality is determined to have occurred based on only a single discrimination operation and the information indicative of the abnormality is stored in the memory device, there are instances where erroneous information ascribable to but a single accidental occurrence of external noise is written into the memory device even though the input and output units are actually devoid of any malfunction. If such erroneous information is written into the memory, an accurate determination as to whether an input or output unit is operating abnormally cannot be made. As a result of this problem, the electronic control unit may operate erroneously as by not implementing the alarm or failure compensation action when an abnormality actually occurs, or by unnecessarily executing the failure compensation action when no abnormality has actually developed.